Low triiodothyronine (T3) concentrations in the presence of normal thyroxine (T4) and TSH levels, referred to as the low T3 syndrome (LT3S), are common. LT3S may be caused by starvation, various non-thyroidal illnesses (NTIs) and some medications. Reverse T3 (rT3) concentrations are elevated in the more severely ill, and they characteristically fail to respond to exogenous levothyroxine (l-T4) therapy. The biochemical abnormalities have been explained on the basis of altered peripheral deiodinase activities. Herein, we report on two patients with hypothyroid symptoms who on testing were found to have LT3S. They were atypical clinically in not having LT3S due to any of the usual causes, had no increased rT3 concentrations, and had a normal negative TSH feedback response to l-T4. One (patient 1) had previously been diagnosed with Hashimoto's autoimmune primary hypothyroidism and was on l-T4 therapy. Both had T4 concentrations in the reference range. TSH levels were elevated in patient 1 and in the reference range in patient 2. Starting or increasing l-T4 doses resulted in no clinical improvement and no increase in T3 levels in spite of a marked increase in T4 levels. It is suggested that in the absence of the usual causes, lack of elevated rT3 levels, response to treatment and intact negative TSH feedback these two patients differ from the usual secondary causes of decreases in deiodinase activity. It is speculated that they may represent primary alterations in deiodinase enzymes possibly due to genetic variations in the deiodinase-encoding genes.
LT3S is commonly found secondary to starvation, NTIs and use of some medications.
Low T3 levels are the result of alterations in the activity of deiodinase enzymes.
LT3S without the usual causes may represent a primary disturbance in deiodinase activity.